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There's a warning out for people who may have recently purchased a used car.

The Bureau of Automotive Repair says some of those cars may not have legally passed the smog test, because of a practice they call clean piping."

"They're cheating and stealing," said the agency's chief, Sherry Mehl. "They will hook up their machine to a car that will pass the smog test, and run that car all day. That's why we call it clean piping."

Mehl said the corrupt technician then sends the state the vehicle identification number of the dirty car, to make it seem as if it passed the test. And used car buyers may be victims.

"I think people are also selling a car that they're desperate to sell, and need to get them smogged, so they pay an extra $100 or $200 and get it done," said Mehl.

Mehl said then when that buyer goes back in two years for the bi-annual smog check, it comes out as a dirty car.

The crime is a felony, and punishable by up to three years in prison. Police arrested 11 people from seven shops in Santa Clara County and the stings are taking place all across the state.

"We take this criminal offense very seriously," said Santa Clara County District Attorney Dolores Carr.

Two of the shops in question visited by NBC Bay Area News are under new management.

The technician in one of those shops said he'd never risk losing his license or going to prison for a few extra dollars.

"I do everything by the book," said technician Long Yoa at the Cal gas station on Keyes Street in San Jose.

The Bureau of Automotive Repair says it gives car owners up to $500 to fix a car that has failed the smog test.